An extra-high pressure mercury lamp has been widely used as, for example, a light source of a projector apparatus for many years. In recent years, with progress of miniaturization of such a projector apparatus, a potable type projector is in widespread use. In even such a miniaturized projector apparatus, a sufficiently bright image is required for a light source, so as to be used daytime. Because of such a background, much more miniaturization and much higher output of a light source installed in such a projector apparatus are studied.
As one of measures of the miniaturization and accomplishment of high output, miniaturization of an electrode arranged in an extra-high pressure discharge lamp or a rod portion which holds an electrode is studied. Moreover, in order to achieve high output, the pressure therein is raised at time of lighting by raising high electric power applied to the electrode and increasing the quantity of mercury which is enclosed in a light emission section. However, in such an extra-high pressure discharge lamp with high electric power in which the electrode or the rod portion which holds the electrode is miniaturized, there is a problem that the electrode or the rod portion which holds the electrode often fractures or is broken. The “break or breakage”, notably occurs in a portion of the electrode or the rod portion which holds the electrode, which is smallest in diameter. Moreover, the “break or breakage” occurs due to an external force applied to the extra-high pressure discharge lamp, such as vibration applied thereto at the time of manufacture, or during transportation thereof.
Japanese Laid Open Patent No. 2007-287387 teaches that in order to increase the mechanical strength of the portion (the smallest diameter portion) where the electrode or the rod portion which holds the electrode is smallest in diameter, the number of crystal grain boundaries of tungsten material which is a constituent of the electrode or the rod holding the electrode is increased to a predetermined number.
FIG. 7 shows an extra-high pressure discharge lamp of prior art. FIG. 7 is a schematic cross sectional view of the extra-high pressure discharge lamp, taken along a plane including the tube axis of the extra-high pressure discharge lamp. The extra-high pressure discharge lamp 100 is equipped with a bulb 110, in which the bulb 110 has an approximately spherical shaped light emission section 111 in which an interior space S is formed, and a pillar-shaped sealing portion 112 connected to both ends of the light emission section 111. While in the interior space S, a pair of electrodes 113 and 114 is arranged facing each other, 0.15 mg/mm3 or more of mercury and halogen gas for performing a halogen cycles are enclosed as light-emitting material. Each of the electrodes 113 and 114 is partially buried in the sealing portion 112, and the electrode 113 is connected to one end of the metallic foil 115 for electric supply. The external lead 116 which projects outward from the sealing portion 112 is connected to the other end of the metallic foil 115. In the extra-high pressure discharge lamp of the prior art, for example, one of the electrodes, that is, the electrode 113 has the smallest diameter portion 117. The number of the grain boundaries which a straight line perpendicular to the electrode central axis crosses in a sectional view taken along a plane including the electrode center of the smallest diameter portion 117, is specified.